The present invention relates to a heat-fixing apparatus adapted to be mounted in image-forming apparatuses such as electrophotographic apparatuses and electrostatic printing apparatuses for fixing a toner image to a paper or other sheet member between a pair of rolls rotating at some contact pressure, at least one of which is provided with a heating means, and more particularly to an improved heat-fixing apparatus whose rolls are always kept clean to prevent the contamination of papers or other image-bearing sheets.
Conventionally known as a fixing apparatus for use in image-forming apparatuses is a heat-fixing apparatus of a heating roll type comprising a heating roll and a pressure roll arranged opposite to each other, the heating roll being constituted by a metal cylinder having good thermal conductivity coated with a non-adherent, heat-resistant layer and containing a heat source such as an infrared lamp, a halogen lamp or a nickel-chromium wire therein, and the pressure roll being constituted by a metal cylinder coated with a heat-resistant, elastic surface layer. In the above heat-fixing apparatus, the fixation of a toner image formed on a sheet is carried out by energizing a heat source to heat the surface of the heating roll to temperatures necessary for fixing of the toner, rotating the heating roll and the pressure roll in contact with each other at a proper contact pressure, and passing the sheet bearing the toner image between the two rolls.
In such a heat-fixing apparatus, it is particularly important to prevent "offset" of toner image borne on the sheet in order to obtain a high-quality fixed image. More specifically, the toner to be fixed to a toner image-bearing sheet member should be prevented from being transferred onto the heating roll partially while passing through a gap between the pressure roll and the heating roll and then adhered to the image-bearing sheet member again.
Conventionally used to prevent offset of the fixed image is a heating roll coated with resins having good release properties such as polytetrafluoroethylene [PTFE], perfluoroalkoxy-tetrafluoroethylene copolymers [PFA], etc. Such a heating roll, however, has insufficient fixing capability because of low elasticity, and also it is likely to incur surface damage. To eliminate these defects, a proposal was made to provide a heating roll coated with a layer of a mixture of a fluorine rubber and a fluorine resin, which is baked to have a fluorine resin layer as an outermost layer [U.S. Pat. No. 4,568,275]. This heating roll, however, is poor in durability. Specifically speaking, because it has a fluorine resin surface layer of only several .mu.m in thickness, the surface layer is worn out by producing only several tens of thousands of copies due to contact with a cleaning member, etc.
Thus, a proposal was made to provide a heating roll having a fluorine resin layer over the above mixture layer [Japanese Patent Laid-Open No. 59-217010].
An attempt was also made to propose/provide a heating roll and a pressure roll, each of which is constituted by a core cylinder coated with a heat-resistant, elastic layer and then with a fluorine resin layer [U.S. Pat. No. 4,219,327, EPA 0186314].
None of these apparatuses using heating rolls having multilayer structures of core-heat resistant, elastic layer-release layer, however, succeeded in preventing toner from being adhered to roll surfaces completely.
As a means for preventing offsetting, various proposals were made. For instance, a cleaning pad constituted by a felt, a blade and a roller, each of which is in pressed contact with a heating roll, were proposed [Japanese Patent Laid-Open Nos. 57-44168, 56-52784 and 57-105760, Japanese Patent Publication No. 51-10108].
However, when the cleaning pad is used, its area in contact with the heating roll is so limited that its service life is too short, making it necessary to exchange it every 2000-3000 copies and also making its maintenance frequent and troublesome. Thus, the cleaning pad cannot effectively be used in a recent high-speed fixing apparatus.
In the systems using the toner-removing roller pressed onto a heating roll, the difference in surface energy between the heating roll and the toner-removing roller is utilized to transfer the toner from the heating roll to the toner-removing roller. However, control of pressing pressure of the toner-removing roller to the heating roll is difficult, and replacement of the surface member of the toner-removing roller is troublesome.
In addition, in the system of using the blade, the heating roll should have a relatively hard or rigid surface, but when the heating roll is coated with a heat-resistant, elastic layer as a surface layer, the blade cannot be used. That is, if the blade is pressed to the heating roll provided with a heat-resistant, elastic surface layer, both side edge portions of the heating roll are deformed in a wavelike manner, making it impossible to fix a toner image with high fidelity to a toner image-bearing sheet member. It also drastically reduces the service life of heat rolls.